A process for the laser welding of a tube with a probe that can be inserted in the tube is known, for example, from EP-A1-0 300 458. The probe described is connected by means of an optical waveguide with an Nd:YAG solid-body laser. The laser beam that emerges from one end of the optical waveguide within the probe is focused on a focal point located outside the probe by means of a lens system consisting of several lenses and a deflecting mirror. The deflecting mirror is inclined at an angle of 45.degree. to the longitudinal axis of the probe and deflects by 90.degree. the laser beam that is focused by the lens system and extends within the probe between the lens system and the deflecting mirror. The deflected laser beam leaves the probe through a cylindrical outlet opening located radially in the housing of the probe. The laser beam that has been focused on the inner surface of the tube that is to be welded thus impinges directly opposite the outlet opening, in the middle, perpendicularly on the inner surface.
In this familiar device, a significant part of the laser light that impinges on the inner surface of the tube is thus reflected back upon itself and, accordingly, into the interior of the probe. This produces an additional thermal stress on the optical components mounted within the probe. Since the outlet opening for the laser beam is located directly opposite the welding area, welding vapor, welding plasma, or--particularly when a pulsed laser is employed--drops released from the melt may rebound on the deflecting mirror and on the outlet opening, thus considerably reducing the useful life of the probe.
One purpose of the present invention is therefore to provide a device and a process for the laser welding of a tube along its inner surface with a probe that can be inserted in the tube, with which a thermal stress on the optical components mounted in the probe and a precipitation of welding vapor on the deflecting mirror and in the vicinity of the outlet opening can be reduced to a large extent.